1. Field of the Invention
The present invention relates to a control method and control circuit for a buck switching regulator, and more particularly, to a control method and control circuit capable of increasing an adaptive range for an input voltage of a buck switching regulator.
2. Description of the Prior Art
DC-DC switching regulators are widely applied in various kinds of power supply devices, wherein the main function of the DC-DC switching regulator is to provide stable DC power for various electronic elements. The DC-DC switching regulator can be divided into buck type, boost type and buck-boost type according to relations between the output voltage and input voltage. A well-controlled switching regulator can retain high power conversion efficiency and provide a stable output voltage in different situations such as different input voltages or different loading currents.
The buck switching regulator is a switching regulator type where the output voltage is less than the input voltage. There are various control methods for the conventional buck switching regulator. One common control method adjusts the duty cycle of a pulse width modulation (PWM) signal to stabilize the output voltage. According to this control method, the output terminal and the input terminal of the buck switching regulator should comply with the law of conservation of energy. Therefore, the duty cycle may be equal to a ratio of the output voltage to the input voltage of the buck switching regulator; the relations between a duty cycle D, an output voltage Vout and an input voltage Vin can be derived as follows:D=Vout/Vin.  (1)
In an ideal case, the input voltage Vin of the buck switching regulator may fall to a voltage level equal to the output voltage Vout. In such a condition, the duty cycle D should be equal to 1, such that the power can be transmitted from the input terminal to the output terminal completely, i.e. the power conversion efficiency is 100%. In practice, when the buck switching regulator is operated, this cannot be achieved because: even if the power transistor is extremely large, a drop out may still be unavoidable; and when the control circuit of the buck switching regulator is operated, the pulse width modulation signal cannot be turned on permanently, so that at least a small off time has to be utilized for buffering. During the off time of the pulse width modulation signal, partial detecting devices may perform over-current detection or other operations. When the input voltage Vin is gradually reduced, the on time of the pulse width modulation signal may increase and the off time will be reduced. If the off time reaches a minimum value while the input voltage Vin still keeps falling, the buck switching regulator may not transmit enough power to the output terminal, such that the output voltage Vout has to fall in accordance with the input voltage Vin, in order to maintain the system balance. As a result, when the input voltage Vin falls to a lower voltage level, the buck switching regulator may not output the stable output voltage Vout continuously. Thus, there is a need to provide a method which allows the buck switching regulator to output a stable output voltage even when the input voltage falls to a lower voltage level, in order to increase an adaptive range for the input voltage of the buck switching regulator.